CH639568A5 - AIR FILTERING ASSEMBLY. - Google Patents

Description

The subject of the present invention is an extremely efficient air filter assembly and more particularly an air filter assembly capable of easily capturing, with very high efficiency, ultra-fine particles (whose diameter is for example 0.1 μm ) suspended in the air, for example to purify the air, collect dust, remove dust and kill bacteria.

Currently, increasingly efficient air filters are required to allow increasingly fine airborne particles, including ultra-fine particles, to be removed.

Especially in factories intended for the manufacture of IC circuits (integrated circuits) and LSI circuits (large-scale integrated circuits), in operating rooms and treatment rooms of hospitals, as well as in rooms for the preparation of drugs and the like, the removal of dust, microbes, etc., floating in the air presents great difficulties; to reduce the introduction of dust and microbes due to the entry of people into the premises, the introduction of devices, etc., as well as for the removal of suspended dust and microbes, air purification systems in which the air supplied to the premises is first filtered, in order to be purified, by very efficient air filters, then is blown from the ceiling (vertical laminar flow system) or through the walls (horizontal laminar flow system).

However, especially in the field of IC and LSI circuit production, there is a trend towards ever smaller dimensions and ever higher densities and precisions, which requires being able to accurately isolate an interval of less than 0.3 µm. There is also concern in hospitals today about microbes smaller than 0.3 µm. Thus the removal of these ultra-fine particles becomes essential. More precisely, in rooms with traditional purified atmosphere, supplied with air by purifiers with HEPA filters (High Efficiency Particulate Air) whose degree of collection or retention is greater than 99.97% for fine particles of order of 0.3 | im in diameter, the purification obtained is not sufficient, because it only enters class 1 or 100 at most (that is to say that the number of particles per cubic foot of air is 1 or 100 respectively according to the standard

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639,568

US 209b) for fine particles with a diameter greater than 0.5 μm, and it is therefore necessary to obtain better purified premises, that is to say super-purified premises of class 1 or 100 for particles fines of less than 0.3 | tm, for example for ultra-fine particles of 0.1 | im.

For the specifications described above, electrostatic precipitation chambers and conventional electrostatic filters are not only expensive to install with the required power sources, but their efficiency in removing dust composed of ultra-fine particles like those described above is insufficient. In addition, when conventional high-efficiency air filters (HEPA filters) must be used, the use of a single filter sheet is not sufficient to remove dust in the case of ultra-fine particles. To overcome the drawbacks described above, attempts have obviously been made to improve the degree of dust retention either by stacking two or more sheets, or by using a single sheet of higher density, but, in both cases, although the degree of retention of the dust was improved, the pressure drop was very great. Thus, not only did the system members such as blowers, pipes, etc. become bulky, but the power consumption was increased, which made this solution unsuitable and insecure for this use. In addition, since conventional high-efficiency air filters rely on particle retention by a mechanical filtering effect, it may be possible to retain fine particles if the gap between the fibers is further reduced and, to retain finer particles, we could possibly make a filter whose pores would have an even smaller diameter. However, even in the above arrangement, the drawbacks described cannot be avoided, the filters tending to clog up with the result, an increase in pressure drops and a shortened service life of the filters.

The object of the invention is the production of a highly efficient improved air filtering assembly without the risk of fouling and substantially eliminating the drawbacks specific to conventional air filters.

According to the present invention, the air filtering assembly is characterized in that it comprises an electrically polarized filtering material, creating a static electric field and a non-electrically polarized filtering material, these two materials being arranged side by side. so as to be successively traversed by the air to be filtered. For example, when stacking a HEPA filter sheet and an electret filter sheet, particles larger than 0.3 | im are collected by the HEPA filter while particles smaller than 0.3 | im are collected by the electret filter, so that a filter is obtained whose collecting power is extremely large and whose pressure drops are small. It should be noted here that, since the electret filter does not retain suspended particles using a fine mechanical sieve, but by attracting them to the surface of fibers using the force of Coulomb resulting from the surface potential of these, it does not fill and the increase in pressure drops is very low, while having a very high retention power.

The electret to be used is generally in the form of a sheet. In the case of a sheet-shaped electret, one of the surfaces thereof can be positively charged and the other surface negatively, or else these surfaces can have both positive and negative charges, such as this is for example the case when the electret consists of woven or nonwoven fabric obtained from previously polarized fibers.

The phenomenon described above is also found with an electret which is not in sheet form, for example in the case where it is in granular form because, due to the Coulomb forces, etc., generated by the potential of surface of the material, the suspended particles which are charged are attracted to the surface of the electret which is charged at the opposite polarity, while the neutral particles are also attracted to neighboring surfaces due to electrostatic induction. Furthermore, the electret used in the present invention can be prepared by incorporating, in a box-shaped element, an electretized tissue or an electretized porous structure, for example non-woven tissue, expanded material or porous cell material. communicating, etc., as well as a granular or similar electret, which is stacked, and zigzag folded, etc., in the case of leaves, as required.

As for conventional filters, for example medium efficiency air filters (for example NB 95 type filters) and high efficiency air filters (HEPA filters), etc., these are generally zigzag folded when their usage.

In the accompanying drawings, given by way of example:

fig. l (a) to l (g) are schematic side sectional views showing different ways of arranging the electret filters and conventional filters relative to an air flow, as well as the configuration of each of the filters;

fig. 2 (a) to 2 (c) are schematic side sectional views showing the arrangement of the filters used in three different filter assemblies, and FIGS. 3 to 8 are schematic side sectional views showing various embodiments intended mainly for premises with a highly purified atmosphere.

In the embodiment shown in FIG. 1, at least one conventional filter 1 is combined with at least one electret filter 2 disposed on the downstream side, in order to use the strengths of the two kinds of filters to obtain a filter assembly having particularly favorable characteristics. Particles with a diameter greater than 0.3 µm are collected by the conventional filter 1 located upstream, while ultra-fine particles are captured by the electret filter 2, as has been confirmed by experiments. These experiments also demonstrated that the pressure drops are smaller when the electret filter is placed on the downstream side. When the conventional filter element 1 is bent in a zigzag, the collecting power can be increased at the cost of a small increase in pressure losses, although this leads to a certain increase in thickness. In fig. 1, (a) shows a variant in which a conventional laminar filter 1 zigzag folded is held between two electret filters 2 laminar flat, (b) shows a variant in which an electret filter 2 laminar flat is arranged downstream of a filter classic 1 laminar zigzag folded, (c) shows a variant in which a flat laminar 2 electret filter is arranged downstream of a classic 1 laminar flat filter, (d) shows a variant in which a laminar zigzag folded electret filter 2 is arranged on the downstream side of a conventional 1 laminar filter folded in the same way in a zigzag, (e) shows a variant in which an electret 2 laminar filter folded in a zigzag is arranged on the downstream side of a conventional flat 1 laminar filter, (f) shows a variant in which a laminar electret filter 2 is placed in intimate contact between two laminar filters 1 folded in a zigzag, and (g) shows a variant in which the conventional laminar filter 1 folded in a zigzag arranged on the side downstream at (0 is replaced by a conventional flat laminar filter. It should be noted here that, in the filter assemblies shown in (f) and (g), the conventional filter 1 located on the downstream side can be modified so as to form only a support element intended to support the filter intermediate electret. In addition, the filter assembly may contain electretized fibers inside a conventional filter. In other words, the filter assembly can, for example, be made of a fabric composed of electretized fibers and non-electretized fibers for a conventional filter. In addition, it is possible to produce a filter assembly in which an electret in powder or granular form is enclosed in a conventional laminar filter.

The improvement in performance obtained using air filters constructed in accordance with the present invention will be described below, with supporting experimental data.

It should be noted that the examples given here are only given by way of illustration of the present invention and without limitation thereof.

Example 1:

Downstream of a HEPA filter 1, laminar and flat, described below, an electret filter 2, laminar and flat, has the properties described below, the arrangement being illustrated in FIG. 2 (a).

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4

HEPA filter

Material Thickness Quantity filtered

Pressure drop at a flow of 10 cm / s

Electret filter:

Material Thickness Quantity filtered

Pressure drop at a flow of 10 cm / s

High efficiency air filter

Glass fibers

0.4 mm

80 gsm

56 mm WG

Polypropylene fibers 5 mm 400 g / m2 7 mm WG

Electret filters:

Material Thickness Quantity filtered

Pressure drop at a flow of 10 cm / s

Example 2:

Between two electret filters 2, laminar and flat, having the properties described below, there is arranged as illustrated in FIG. 2 (b) a HEPA filter 1, also laminar and flat, like the one described below.

HEPA filter: like that of Example 1

Polypropylene fibers

4 mm 300 g / m2

5 mm WG

Example 3:

Upstream of two electret filters 2, laminar and flat, such as those described below, an NB 95 filter, laminar and also flat, having the properties described below, is arranged, as shown in FIG. 2 (c).

NB 95 filter: (NB = National (US) Bureau of Standards)

Material Glass fibers

0.4 mm thick

Filtered quantity 90 g / m2

Pressure drop at a flow of 10 cm / s 11 mm WG

Electret filters: like those of Example 1.

The results compared are shown in the table below.

Examples

Classic filters

Electret filters

1

2

3

HEPA 1 sheet

HEPA 2 sheets

NB 95 1 sheet

Quantity

filtered 400 g / m2

Quantity

filtered 300 g / m2

Pressure drop 10 cm / s (mm WG)

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66

26

56

112

16

7

5

Yield (%)

0.111 particles | im

0.00

0.00

0.00

0.20

0.09

45.2

0.01

0.93

0.29 particles | im

0.00

0.00

0.00

0.01

0.00

13.4

0.00

0.06

The above yields are determined by establishing the percentage ratio between the number of particles downstream and upstream of the filter assembly in a volume of 100 cm3 and at a speed of 10 cm / s. The instrument used for the measurement was a PMS (Particles Measuring System, Inc.) laser spec-trometer for aerosols.

It is clear from the results of the experiments indicated above that a filter assembly according to the invention is far superior to the conventional filters or to the electret filters alone. Although, in the above examples, the lower value of the filtered weight of the electret filter was fixed at 300 g / m2, it was also perfectly possible to remove particles of 0.111 μm in size with electret filters filtering a lower weight to that indicated above, for example 200 g / m2.

It should be noted here that, among the instruments making it possible to measure the number of fine particles, the PMS laser spectrometer used, capable of measuring particles of 0.111 (im, is the most efficient and, given that particles of diameter smaller than that indicated cannot be measured, it must be admitted that the particles of diameter less than 0.111 μm have been removed in large quantity, since those whose diameter is 0.111 μm have been completely removed by the filtering assembly according to the invention, as is clear from the above data.

We will now describe the construction of the most used filter assemblies for premises whose atmosphere must be purified to a high degree.

The arrangement shown in fig. 3 (a) is an embodiment corresponding to the arrangement shown in FIG. l (d) and it is constructed so that the conventional high performance filter 1 and the electret filter 2, laminar and zigzag folded, are arranged respectively on the upstream side and on the downstream side as shown by being fixed by their edges to a chassis 3 by appropriate means.

The arrangement shown in fig. 3 (b) is generally similar to that of FIG. 3 (a) and is constructed so that the conventional high performance filter 1 disposed on the upstream side and the electret filter 2 disposed on the downstream side are both zigzagged around spacers 4 of aluminum or the like arranged one by one , the edges of the filters being fixed to the frame 3 by appropriate means.

By blowing air through air filters constructed as above and arranged in the ventilation opening of the room at atmospheres-50 pure sphere, the incoming air, polluted, is practically freed of dust and microbes so that it is highly purified air which is blown into the room, thus guaranteeing in the room a purified atmosphere to a high degree.

55 Fig. 4 shows a variant of the arrangement shown in FIG. 3 (b). In this fig. 4, the conventional filter 1 and the electret filter 2, both laminar and zigzag folded, are arranged perpendicular to one another, FIG. 4 (a) showing the arrangement in lateral elevation and FIG. 4 (b) showing the plan arrangement with the frame m shown in section.

In the arrangement shown in fig. 5 (a), there is, on the upstream side, a conventional laminar filter 1, of high performance, zigzag folded around spacers 4 regularly spaced, the filter being fixed by its edges to a frame 3 by suitable means as in 65 fig. 3 (b), while, on the downstream side, there is an electret filter, laminar and zigzag folded, fixed by its edges to the frame 3, by suitable means, substantially as in FIG. 3 (a). The arrangement shown in fig. 5 (b), on the other hand, is constructed so that,

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on the upstream side, there is a conventional high performance filter, laminar and zigzag folded, fixed by its edges to the frame 3 by suitable means, substantially in the same way as in FIG. 3 (a), while, on the downstream side, there is a laminar electret filter, folded in a zigzag around regularly spaced spacers 4, the filter being fixed by its edges to a frame 3 by suitable means substantially of the same so that in fig. 3 (b).

In the arrangement shown in fig. 6, there is, on the upstream side, a conventional laminar filter 1, with high performance, which is zigzag folded around spacers 4 of aluminum or the like regularly spaced apart, the filter being fixed by its edges to a frame 3 by suitable means as in fig. 3 (b), while, on the downstream side, there is an electret filter 2 which is formed by filling the chassis 3 to the desired density with electretized cotton fibers. The reference index 5 designates the appropriate number of support members necessary to support these fibers, etc., these members being for example in the form of a comb, made of wood, plastic or metal, or in the form of a trellis , or even a porous plate.

In the arrangement shown in fig. 7, a conventional filter 1 of high performance, folded in a zigzag, is arranged in a frame 3 by forming a new zigzag, the filter being fixed by its peripheral edges to the frame 3, the surfaces of the folds of the filter being substantially perpendicular to the air flow direction. In addition, an electret filter 2 of cotton fibers, etc., electretized, is introduced at the desired density into the wedge-shaped spaces formed by the filter 1 on the downstream side. These fibers and others are leaning against support elements 5 in the form of a comb, made of wood, plastic or metal, in the form of a trellis, or in the form of a porous plate, etc.

In the arrangement shown in fig. 8, a conventional filter 1 of high performance, folded in a zigzag, is housed in a frame 3 by forming a new zigzag, the filter being fixed by its peripheral edges to the frame 3, the faces of the folds of this filter also being substantially perpendicular to the air flow direction. In addition, an electret filter 2 of cotton fibers, etc., electretized, is introduced at the desired density into the spaces formed along the filter 1 downstream thereof. In this arrangement also, the fibers of the electret filter 2 rest against support elements 5, in the form of a comb, made of wood, plastic or metal, in the form of a lattice, or in the form of a porous plate, etc. Since the rear parts of the support elements 5 provide wedge-shaped spaces, less electrically polarized material is required than in the case of FIG. 7, the filter assembly described above can be produced more economically, and this without reducing performance.

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2 sheets of drawings

Claims (17)

639,568 2 1. Air filter assembly, characterized in that it comprises an electrically polarized filter material creating a static electric field and a non-electrically polarized filter material, these two materials being arranged side by side so as to be passed through successively by the air to be filtered. 2. Air filter assembly according to claim 1, characterized in that the electrically polarized filter material comprises a fibrous material, woven or not, porous, expanded and / or granular, and in that the non-polarized filter material electrically comprises a laminar air filter material made of glass fibers, 0.4 mm thick, capable of filtering 80 g / m2 with a pressure drop of 56 mm WG at a flow of 10 cm / s and / or a laminar air filter material made of glass fibers 0.4 mm thick, capable of filtering 90 g / m2 with a pressure drop of 11 mm WG at a flow of 10 cm / s. 3. Air filter assembly according to one of claims 1 or 2, characterized in that it comprises at least one sheet and / or at least one piece of electrically polarized fibrous or porous material and at least one sheet and / or at least one piece of non-electrically polarized filter material. 4. Air filter assembly according to one of claims 1 to 3, characterized in that the electrically polarized filter material is disposed on the upstream side and the non-electrically polarized filter material is disposed on the downstream side of the assembly . 5. Air filter assembly according to one of claims 1 to 3, characterized in that the non-electrically polarized filter material is disposed on the upstream side and the electrically polarized filter material is disposed on the downstream side of the assembly . 6. Air filter assembly according to one of claims 1 to 3, characterized in that one of the two filter materials is disposed on either side of the other. 7. Air filter assembly according to claim 5, characterized in that the non-electrically polarized filter material is arranged in a zigzag around spacers in the upstream part of a frame, and in that the electrically polarized material is arranged in a zigzag around spacers in the downstream part of this frame, the two filtering materials being in the form of a sheet. 8. Air filter assembly according to claim 5, characterized in that the non-electrically polarized filter material is in the form of a sheet and is arranged in a zigzag around spacers in the upstream part of a frame, and in that that at least one sheet of electrically polarized material is supported in the downstream part of the frame perpendicular to the direction of air flow. 9. Air filter assembly according to claim 5, characterized in that the non-electrically polarized filter material is in the form of a zigzag and is arranged in a frame forming a new zigzag, and in that the electrically polarized material comes fill the wedge-shaped spaces on the downstream side with non-electrically polarized material. 10. Air filter assembly according to claim 5, characterized in that the non-electrically polarized filter material is in the form of a zigzag and is arranged in a frame forming a new zigzag, and in that the electrically polarized material is introduced into the spaces on the downstream side of the material not electrically polarized along the inclined planes of the new zigzag. 11. Air filter assembly according to claim 5, characterized in that the non-electrically polarized filter material is arranged in a zigzag around spacers in the upstream part of a frame, and in that the electrically polarized material is arranged in a zigzag pattern on the downstream side, the two filtering materials being in the form of a sheet. 12. Air filter assembly according to claim 5, characterized in that the non-electrically polarized filter material is arranged in a zigzag in the upstream part of a frame, and in that the electrically polarized filter material is arranged in zigzag on the downstream side, the two filter materials being in sheet form. 13. Air filter assembly according to claim 5, characterized in that the non-electrically polarized filter material is arranged in a zigzag in the upstream part of a frame, and in that the electrically polarized filter material is arranged in zigzag around spacers on the downstream side, the two filter materials being in sheet form. 14. Air filter assembly according to claim 1, characterized in that the electrically non-polarized material consists of a sheet of laminar air filter material made of glass fibers, 0.4 mm thick, capable of to filter 80 g / m2 with a pressure drop of 56 mm WG at a flow of 10 cm / s, substantially free of electric charge, and that the electrically polarized material consists of a sheet of dielectric material, one of the surfaces of which has a positive charge and the other has a negative charge. 15. An air filtering assembly according to claim 14, characterized in that the surface with positive charge and the surface with negative charge of the sheet of dielectric material comprise fibers respectively charged with positive and negative polarities. 16. Air filter assembly according to claim 14, characterized in that the sheet of dielectric material is made of a single material, preferably polypropylene. 17. An air filtering assembly according to claim 16, caraté-risé in that at least one of the sheets is arranged in a zigzag.